Blood pressure transducers or probes, consisting of miniature sensors mounted on the distal end of a catheter, are commonly used to measure blood pressure in patients. The advantages of this type of blood pressure sensing include greatly increased signal fidelity and frequency response. Numerous methods of fabricating the sensors have been developed. One common method uses a piezoresistive technique, where one or more resistors are deposited on a pressure sensitive silicon diaphragm. These resistors are usually connected in a Wheatstone bridge configuration, although other configurations are also used.
A common method of fabricating piezoresistive pressure sensors typically includes the placement of an additional temperature sensing element on the same chip in close proximity to the pressure sensing resistors. Temperature sensing devices are needed due to the tendency of piezoresistive bridge sensors to exhibit changes in output with increasing or decreasing temperature. Their function is either to act in a manner which opposes the temperature characteristics of the sensor, or to provide a signal, proportional to the chip temperature, to external circuitry which corrects the temperature characteristics of the sensor signal outputs.
In addition to the temperature characteristics described above, the sensors normally have unwanted fixed offset signals at zero pressure, of varying magnitude from device to device, and also have variations in their sensitivity to pressure. These variations in offset and sensitivity, as well as the variations in the temperature characteristics, must be individually adjusted in each device by a process known as "trimming". The name derives from the most common process of performing this adjustment, namely the use of a laser to change the resistance of particular sensor elements, or of other passive or active circuit elements connected in a network about the sensor to effect the desired changes at the sensor's signal outputs.
The trimming process can include the programming of a digital memory device as a part of a sensor assembly, said sensor assembly including the sensor and mechanical and electrical connections required for operation. Information in the digital memory device is retrieved later by a digital processing circuit designed to receive the signal output from the sensor at the time of use, and mathematically removes the unwanted variations in the signal outputs based upon the information stored in the digital memory device.
A particular application for catheter tip sensing requires that the size of the sensor, and also the number of wire connections to the diaphragm be kept minimum due to overall size constraints of that application. Little room exists for additional circuit elements in close proximity to the pressure sensor and the number of electrical connections to the sensor is limited. It is difficult to integrate digital memory elements on the same silicon chip as the pressure sensor, due to incompatibilities in processing techniques.
In the manufacture of medical transducers, there is an increasing trend toward disposable devices with lower risk of infection and which are convenient for clinics and hospitals to use, since there is no need to sterilize the devices. In order for disposable devices to be practical they must be inexpensive to manufacture. In the case of a disposable transducer, this means that the cost of the circuit elements (sensor and trim circuit) in the disposable portion of the product must be minimized, and the processing time to perform the trim operation must be kept short.
Temperature compensation methods for pressure transducers have been described in U.S. Pat. Nos. 3,841,150, 3,956,927, 3,836,796, 4,556,807, 3,646,815, 3,841,150, 4,196,382 and 4,320,664. Separate temperature sensing devices on the chip achieve compensation. Methods which involve the use of digital or microprocessor computing circuits to compensate the pressure signal have been described in U.S. Pat. Nos. 4,226,125, 4,399,515, 4,446,527, 4,598,381, 4,765,188, and 4,858,615. Those methods and circuits reduce the time required to trim, since the error terms inherent in the sensor are stored in an easily programmed non-volatile digital memory. Since the error terms are unique to each sensor, this requires that a digital memory device be programmed and be a part of each sensor assembly. This increases the cost and the number of electrical connections necessary between the transducer/memory assembly and its receiving amplifier circuit. All of these techniques also require an independent temperature sensing element or reference element in addition to the pressure sensor.
U.S. Pat. No. 4,715,003 describes a technique of exciting the bridge with a constant current source, and using the voltage at the power input to the bridge as a direct indication of bridge temperature, without additional temperature sensing elements. This technique however also relies on the use of digital circuitry, and a digital memory device programmed with unique correction constants, and is subject to the limitations described. Other techniques for compensating piezoresistive bridge sensors use passive resistor networks having negligible temperature coefficients with respect to the temperature coefficient of the bridge sensor resistors. As a result, the placement of the network close to the bridge sensor is not of concern. Resistor networks are connected to the bridge in various series/shunt combinations, with particular values adjusted to achieve the desired result at the signal outputs of the bridge.
Networks, when combined with the resistors in the bridge, form complex circuits which cannot be easily solved to determine the appropriate resistance values for a given amount of offset and temperature compensation. It is a property of networks that an adjustment for temperature compensation affects the fixed offset or gain at the signal outputs, and vice versa. As a result, the trimming of these devices is an iterative process, in that the response of the sensor must be reverified one or more times during trimming to achieve the desired correction.
U.S. Pat. No. 3,447,362 discloses a technique as described in the two previous paragraphs, where the interactions between error sources during the trim process are minimized, and the resistance values required for compensation can be determined by algebraic formula. This technique requires an open, or broken, connection at one of the four bridge nodes, so that resistance is inserted within two of the bridge arms. This requires an additional wire connection on the sensor, adding to fabrication costs and increasing the surface area of the sensor needed for wire terminations. The need exists for a simple analog temperature compensation technique which minimizes the number of unique resistors and adjustments necessary to achieve compensation but does not require additional temperature sensing elements or special sensor connections and for a method that can be performed in a fast and noniterative manner.